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Atmospheric ducting is a mode of propagation of electromagnetic radiation, usually in the lower layers of Earth’s atmosphere, where the waves are bent by atmospheric refraction...
...The reduced refractive index due to lower densities at the higher altitudes in the Earth's atmosphere bends the signals back toward the Earth. Signals in a higher refractive index layer, i.e., duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material. In some weather conditions, such as inversion layers, density changes so rapidly that waves are guided around the curvature of the earth at constant altitude..Source:Wikipedia
Whistlers aren't utilities, or man-made signals at all, but they're interesting. These audio-frequency electrical sweeps are believed to be generated by distant lightning interacting with the ionosphere. They are best heard by special direct-conversion receivers, though a standard audio amp will work if noise permits, which it usually doesn't. Whistlers are spooky. Something like 10 different sweeps are visible in this unusually large one. Notice how the amplitude builds as they kick in, then drop off. Several university professors have made very good recordings of whistlers and other atmospheric electrical phenomena, and these are available online. Source: Ionospheric Whistlers
Magnetospheric ducts that guide very-low-frequency (VLF) radiation are subject to spontaneous intensification by a linear instability that arises from the coupling between the magnetosphere and the ionosphere. The elements of this instability comprise a localized enhancement of electron precipitation and a field-aligned column of enhanced plasma density, each in turn causing the other. This interaction can create fully developed plasmapheric ducts spontaneously from infinitesimal fluctuations. A whistler duct thus formed attains saturation (nonlinear stabilization) by virtue of ionospheric recombination and the bounded supply of electrons available for precipitation. (Author) Source: oai.dtic.mil...
The propagation of electromagnetic waves through the ionosphere in the presence of irregularities is differently affected in different frequency ranges. Very high frequency waves are scattered whereas very low frequency waves are guided through the ducts formed by the irregularities under suitable conditions.
Source: Scintillation Producing and Whistler Ducting Ionospheric Irregularities
The Ionospheric Research Instrument (IRI), a high power transmitter facility operating in the High Frequency (HF) range. The IRI will be used to temporarily excite a limited area of the ionosphere for scientific study. Source: HAARP Website
ELF/VLF radio waves penetrate deeply beneath the surface of the earth and interact with the geologic structure of the earth. This interaction induces secondary fields with measurable effect at and above the surface of the earth. Proper understanding of the physics of the generation and propagation of ELF/VLF waves and their interactions with earth materials will allow these waves to be used for applications such as sub-surface communications and exploration of the subsurface geological structure. The research called for under this effort is to assess the viability of exploiting the concept of electromagnetic induction to detect and image subterranean features such as tunnels, bunkers, and other potential military targets.
Several distinct methods for ELF/VLF generation are available to support these efforts. Proposers are encouraged to consider including the controlled ELF/VLF sources provided by the 960KW HF transmitter of the HAARP, presently under construction outside Gakona, Alaska and the HIPAS facility located near Fairbanks, Alaska. PL/GPS is the program manager for the HAARP facility. The Office of Naval Research controls the HIPAS facility. Both sites will be available to support the research efforts under this PRDA. Source: Federation of American Scientists
Appalachian structure exhibits large thrust faults; horizontal breaks along which one sheet of rocks moves over top of another sheet of rocks, often for miles. Typically the sheets of rock are hundreds to thousands of feet thick, mountain size blocks of rock that dwarf us. It is possible to have a single, isolated thrust fault, but more commonly they come in sets that may include dozens of individual faults... Source: James Madison University Geography
One of the research topics of present interest is the generation of ELF/VLF waves through HF heating of the auroral electrojet, between 70 and 120 km altitude. Potential DoD applications of this the intense current system that flows in the auroral regions arines at ELF frequencies and to use the generated waves to research are to provide communications to submerged subm locate and probe underground structures Source: Umran Inan/Stanford University
A powerful surface acoustic wave can transport charge along a quasi-one-dimensional channel defined in a piezoelectric semiconductor in packets confined to the minima of the wave’s electrostatic potential. The interaction between electrons can suppress fluctuation in the number of electrons in a packet, resulting in a current that is proportional to the surface acoustic wave (SAW) frequency. This effect has been observed experimentally and is presently being considered as a possible route towards a standard of electric current. Here we present further study of this acoustic charge transport. The main finding is that a weak counterpropagating SAW beam can be used to improve the precision of the current delivered by the device.
It is widely known that humans can hear as low as 20khz. VLFrequencies are typically within a range of 3khz - 30khz. Other websites like this one Auroral Chorus claim the maximum is 10khz, which would be too low for us to hear. The author states that whistlers are caused by lightning strikes that pass through the ducts in the magnetosphere and cause audio waves in VLF. Although these natural whistlers cannot be heard by the human ear, there is nothing in his article that states a man-made VLF could not similarly pass through a man-made duct, and that said man-made VLF could not be between 20khz-30khz.
Band ITU Frequency Waveform Name Band Range Name
VLF 4 3 - 30 KHz Myriametric
LF 5 30 - 300 KHz Kilometric
MF 6 300 - 3000 KHz Hectometric
HF 7 3 - 30 MHz Decametric
VHF 8 30 - 300 MHz Metric
UHF 9 300 - 3000 MHz Decimetric
SHF 10 3 - 30 GHz Centimetric
EHF 11 30 - 300 GHz Millimetric
(IR) 12 300 - 3000 GHz Decimillimetric
Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 kHz to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals are used, such as for radio navigation. Also known as the myriametre band or myriametre wave as the wavelengths range from ten to one myriametres (an obsolete metric unit equal to 10 kilometres). Source: Wikipedia